Use the search button (looks like a magnifying glass on the bottom left).

You may need to press this several times as secret doors will usually not show up until around 3-6 presses of the search button. Obviously you cant do this on every wall as you would be there for ever and probably starve

Generally the dungeon maps work on a 3x3 grid pattern with a minimum of 6 rooms/maximum of 9 rooms, dead end hallways and mazes count as rooms and no wall has more than one door. Use this knowledge to your advantage, by calculating which walls to search, thus cutting down on your search time. So if you can see 8 rooms with a gap in the top right hand side ( see number sequence/diagram below, remembering the 3x3 grid rule) there is probably only two walls worth searching - the top wall of room 6 and the right wall of room 2.

There is a better explanation somewhere, but i cannot find the damn thing